The present invention relates to a tire with radial carcass reinforcement anchored on either side to at least one bead wire and having a crown reinforcement formed of at least two so-called working plies, superposed and made of wires or cables which are parallel in each ply and are crossed from one ply to the next, forming angles of at most 45xc2x0 in absolute value with the circumferential direction of the tire.
It relates more particularly to a tire of the xe2x80x9cheavy vehiclexe2x80x9d type, the ratio of the height H above rim to the maximum axial width S of which is at most 0.80, and which is intended to be fitted on a medium- or high-tonnage vehicle, such as a lorry, bus, trailer, etc.
Some current tires, called xe2x80x9chighwayxe2x80x9d tires, are intended to travel at high speed and on longer and longer journeys, owing to the improvement in the road network and the growth of the motorway network throughout the world. All the conditions under which such a tire is required to travel without doubt make it possible to increase the number of kilometers traveled, the wear of the tire being less; on the other hand, the endurance of the latter and in particular of the crown reinforcement is adversely affected.
The lack of endurance relates both to the fatigue resistance of the crown plies, and in particular the resistance to separation between ends of plies, and to the fatigue resistance of the cables of the portion of carcass reinforcement located beneath the crown reinforcement, the first deficiency being greatly influenced by the operating temperature at the edges of the working plies, whether travelling in a straight line or under drift.
A first solution has been described in French Application FR 2 728 510, and proposes arranging, firstly between the carcass reinforcement and the crown reinforcement working ply radially closest to the axis of rotation, an axially continuous ply formed of inextensible metal cables forming an angle of at least 60xc2x0 with the circumferential direction and the axial width of which is at least equal to the axial width of the shortest working crown ply, and, secondly between the two working crown plies, an additional ply formed of metallic elements oriented substantially parallel to the circumferential direction, the axial width of said ply being at least 0.7 S0.
The problems relating to the separation between working plies and the fatigue resistance of the carcass reinforcement cables have been solved, and the operating temperatures reduced; on the other hand, prolonged travel of the tires thus constructed has caused fatigue breaking of the cables of the additional ply, and more particularly of the edges of said ply, whether or not the so-called triangulation ply is present.
It is always possible to change the reinforcement elements, and in particular to select cables of a different construction or cables of greater tensile strength. The solution above, while admittedly simple, is still costly.
Thus, in order to overcome these above new drawbacks and to improve the endurance of the crown reinforcement of the type of tire in question, French Application FR 97/14011, which has not been published to date, has chosen another solution, and proposes, on either side of the equatorial plane and in the immediate axial extension of the additional ply of reinforcements which are substantially parallel to the circumferential direction, to couple, over a certain axial distance, the two working crown plies formed of reinforcement elements crossed from one ply to the next, then to decouple them by profiled members of rubber mix at least over the remainder of the width common to the said two working plies.
The fatigue strength of the circumferential elements is not optimal, unless the minimum density of the elements at the edges of the ply and a minimum rupture resistance of said elements are respected, which involves a high cost price for material.
In order to improve the endurance of the crown reinforcement of the type of tire in question, without being faced with problems of fatigue of the reinforcement elements, French application FR 98/06000 radically modifies the orientation of the inextensible reinforcement elements of the additional ply which is arranged radially between said working plies, said elements then being radial.
The shearing stresses between the two working crown plies are very great, more particularly in the case of coupling of said two working plies, which results in delamination between the plies as the tire becomes fatigued. In order to overcome the above disadvantages and to improve the endurance of the crown reinforcement of the type of tire in question, the invention proposes ingeniously to reconcile the advantages of radial orientation with those of circumferential orientation of the reinforcements elements of the additional ply located radially between the two working crown plies.
According to a first variant, the tire according to the invention, with radial carcass reinforcement of maximum axial width S0, comprising a crown reinforcement formed of at least two working crown plies of inextensible reinforcement elements, crossed from one ply to the other, forming angles of between 10xc2x0 and 45xc2x0 with the circumferential direction, said plies having axial widths L32, L34 at least equal to 80% of the width S0, is characterized in that an additional ply formed of at least one ply of reinforcement elements, of width L33 less by at least 15% of the width S0 than the width L32 (L33) of the least wide working ply, arranged radially between said working plies, is axially composed of three parts, a central part in the form of a ply formed of inextensible, substantially radial, metallic reinforcement elements, said ply having an axial width Lxe2x80x233 equal to at least 45% of the width S0, and two lateral parts in the form of strips, each formed of circumferential elastic metallic reinforcement elements, the modulus of elasticity upon traction per unit of width of a lateral strip being at most equal to the modulus of elasticity upon traction, measured under the same conditions, of the most extensible working ply, and the width Lxe2x80x333 of each strip being at most 10% of the width S0.
xe2x80x9cInextensible elementxe2x80x9d is to be understood to mean an element, cable or monofilament which has a relative elongation of less than 0.2% when subjected to a tensile force equal to 10% of the breaking load. In the case of the tire in question, the inextensible reinforcement elements are preferably inextensible metal cables made of steel.
Metallic elements oriented substantially parallel to the circumferential direction are elements which form angles within the range +2.5xc2x0, xe2x88x922.5xc2x0 of said 0xc2x0 direction.
Substantially radial reinforcement elements, cords or cables are elements which form angles within the range +5xc2x0, xe2x88x925xc2x0 of the 0xc2x0 meridian direction.
Metallic reinforcement elements are said to be elastic if they have a relative elongation greater than 2% when they are subjected to a tensile force equal to 10% of their breaking load. They have a curve of tensile stress as a function of the relative elongation which has shallow gradients for the slight elongations and a substantially constant, steep gradient for the greater elongations, the change of gradient taking place in a range of relative elongation of between 0.2% and 0.8%. Owing to this, said elements may be referred to as xe2x80x9cbimodularxe2x80x9d elements.
A modulus of elasticity E upon traction of a ply per unit of width results from the tensile stress "sgr" exerted in the direction of the reinforcement elements and over a unit of width to obtain a relative elongation xcex5. xe2x80x9cElasticity modulus of the lateral part of the additional ply which is at most equal to the same modulus of the most extensible working plyxe2x80x9d is to be understood to mean that the modulus of said part of the additional ply, whatever the relative elongation, is at most equal to the modulus of the most extensible working ply whatever the relative elongation, the most extensible ply being the ply which, for each value of tensile stress, has a relative elongation greater than that of the other ply for the same stress.
Advantageously, the modulus of the lateral part of the additional ply will be such that it is low for a low relative elongation of between 0% and 0.5%, and at most equal to the greatest modulus of elasticity upon traction of the most extensible working ply, for relative elongations greater than 0.5%, said moduli of elasticity being approximately equal, for a given relative elongation xcex5, to the products of the tangent modulus of elasticity of the reinforcement elements for said elongation xcex5 and the volume fraction of metal in the ply.
In a second variant, the lateral parts of the additional plies may also be formed of circumferentially inextensible metallic elements cut so as to form sections of a length very much less than the circumference of the least long ply, but preferably greater than 0.1 times said circumference, the cuts between sections being axially offset relative to each other. Such an embodiment makes it possible to impart, in simple manner, to the lateral parts of the additional ply a modulus which can easily be adjusted (by selecting the gaps between sections of the same series), but which in all cases is less than the modulus of the ply formed of the same metallic elements, but continuous ones, the modulus of the additional ply being measured on a vulcanized ply of cut elements, taken from the tire.
In a third variant for obtaining a lateral strip having a lower tensile modulus than the tensile modulus of the most extensible working ply, it is advantageous to use as reinforcement elements for said lateral part undulating metallic elements of circumferential orientation, the ratio a/xcex of the amplitude of undulation to wavelength being at most 0.09.
In the last two variants cited, the metallic elements are preferably steel cables.
Preferably, the working plies, on either side of the equatorial plane and in the immediate axial extension of the additional ply, are coupled over an axial distance 1 at least equal to 3.5% of the width S0, then being decoupled by profiled members of rubber mix at least over the remainder of the width common to said two working plies, the presence of said couplings also permitting the reduction of the tensile stresses acting on the circumferential cables of the edge located closest to the coupling.
The thickness of the decoupling profiled members between working plies, measured level with the ends of the least wide working ply, will be at least equal to two millimeters, and preferably greater than 2.5 mm.
xe2x80x9cCoupled pliesxe2x80x9d is to be understood to mean plies, the respective reinforcement elements of which are separated radially by at most 1.5 mm, said thickness of rubber being measured radially between the respectively upper and lower generatrices of said reinforcement elements.
The working plies generally are of unequal axial widths. Whether the radially outermost working ply is axially less wide than the radially innermost working ply, or whether said radially outermost ply is axially wider than the radially innermost working ply, it is then advantageous for the crown reinforcement to be finished off radially to the outside by at least one additional ply, referred to as a protective ply, of so-called elastic reinforcement elements, oriented relative to the circumferential direction at an angle of between 10xc2x0 and 45xc2x0 in the same direction as the angle formed by the inextensible elements of the working ply radially adjacent thereto.
An elastic reinforcement element for a protective ply meets the same definition as previously, and has a relative elongation of greater than 2% when subjected to a tensile force of 10% of the breaking load. Said elements are also metal cables made of steel.
The protective ply may have an axial width less than the axial width of the least wide working ply, but advantageously sufficient totally to cover the zone of coupling between the two working crown plies, and all the more advantageously since the tread of the tire in question comprises a circumferential or quasi-circumferential groove axially arranged radially on the zone of coupling between the two working plies. Said protective ply may also have an axial width greater than the axial width of the least wide working ply, such that it covers the edges of the least wide working ply and, in the case of the radially upper ply as being the least wide, such that it is coupled, in the axial extension of the additional reinforcement with the widest working crown ply over an axial distance of at least 2% of the width S0, then being decoupled, axially to the outside, from said widest working ply by profiled members of a thickness of at least 2 mm. The protective ply formed of elastic reinforcement elements may, in the case referred to above, be firstly possibly decoupled from the edges of said least wide working ply by profiled members of a thickness substantially less than the thickness of the profiled members separating the edges of the two working plies, and secondly have an axial width less or greater than the axial width of the widest crown ply.
Whatever the solution adopted, the crown reinforcement may be finished off, radially to the inside between the carcass reinforcement and the radially inner working ply closest to said carcass reinforcement, by a triangulation ply of inextensible metallic reinforcement elements made of steel, forming with the circumferential direction an angle greater than 60xc2x0 and of the same direction as that of the angle formed by the reinforcement elements of the ply radially closest to the carcass reinforcement. Said triangulation ply may have an axial width less than said least wide working ply, but may also have the necessary width sufficient for said ply to be able to be coupled with another ply, be it with the widest working ply or the protective ply radially above the working plies, or with the widest working ply.
The characteristics and advantages of the invention will be better understood with the aid of the following description, which relates to the drawing, which illustrates an example of embodiment in non-limitative manner.